The present invention relates to packaging films. In particular, the present invention relates to biaxially stretched, heat shrinkable films made of copolymers of polyethylene.
Polyethylene is the name for a polymer whose basic structure is characterized by the chain --CH.sub.2 CH.sub.2).sub.n. Polyethylene homopolymer is generally described as being a solid which has a partially amorphous phase and partially crystalline phase with a density of between 0.915 to 0.970 g/cm.sup.3. The relative crystallinity of polyethylene is known to affect its physical properties. The amorphous phase imparts flexibility and high impact strength while the crystalline phase imparts a high softening temperature and rigidity.
Unsubstituted polyethylene is generally referred to as high density homopolymer and has a crystallinity of 70 to 90 percent with a density between about 0.96 to 0.97 g/cm.sup.3. Most commercially utilized polyethylenes are not unsubstituted homopolymer but instead have C.sub.2 -C.sub.8 alkyl groups attached to the basic chain. These substituted polyethylenes are also known as branched chain polyethylenes. Also, commercially available polyethylenes frequently include other substituent groups produced by copolymerization. Branching with alkyl groups generally reduces crystallinity, density and melting point. The density of polyethylene is recognized as being closely connected to the crystallinity. The physical properties of commercially available polyethylenes are also affected by average molecular weight and molecular weight distribution, branching length and type of substituents.
People skilled in the art generally refer to several broad categories of polymers and copolymers as "polyethylene." Placement of a particular polymer into one of these categories of "polyethylene" is frequently based upon the density of the "polyethylene" and often by additional reference to the process by which it was made since the process often determines the degree of branching, crystallinity and density. In general, the nomenclature used is nonspecific to a compound but refers instead to a range of compositions. This range often includes both homopolymers and copolymers.
For example, "high density" polyethylene (HDPE) is ordinarily used in the art to refer to both (a) homopolymers of densities between about 0.960 to 0.970 g/cm.sup.3 and (b) copolymers of ethylene and an alpha-olefin (usually 1-butene or 1-hexene) which have densities between 0.940 and 0.958 g/cm.sup.3. HDPE includes polymers made with Ziegler or Phillips type catalysts and is also said to include high molecular weight "polyethylenes." In contrast to HDPE, whose polymer chain has some branching, are "ultra high molecular weight polyethylenes" which are essentially unbranched specialty polymers having a much higher molecular weight than the high molecular weight HDPE.
Hereinafter, the term "polyethylene" will be used (unless indicated otherwise) to refer to ethylene homopolymers as well as copolymers of ethylene with alpha-olefins and the term will be used without regard to the presence or absence of substituent branch groups.
Another broad grouping of polyethylene is "high pressure, low density polyethylene" (LDPE). The polyethylene industry began in the 1930's as a result of the discovery of a commercial process for producing LDPE by Imperial Chemical Industries, Ltd. researchers. LDPE is used to denominate branched homopolymers having densities between 0.915 and 0.930 g/cm.sup.3 as well as copolymers containing polar groups resulting from copolymerization e.g. with vinyl acetate or ethyl acrylate. LDPEs typically contain long branches off the main chain (often termed "backbone") with alkyl substituents of 2 to 8 carbon atoms.
In the 1970's a new grouping of polyethylene was commercialized--Linear Low Density Polyethylene (LLDPE). Only copolymers of ethylene with alpha-olefins are in this group, LLDPEs are presently recognized by those skilled in the art as having densities from 0.915 to .940 g/cm.sup.3. The alpha-olefin utilized is usually 1-butene, 1-hexene, or 1-octene and Ziegler-type catalysts are usually employed (although Phillips catalysts are also used to produce LLDPE having densities at the higher end of the range).
In the 1980's yet another grouping of polyethylene has come into prominence--Very Low Density Polyethylene (VLDPE) which is also called "Ultra Low Density Polyethylene" (ULDPE). This grouping like LLDPEs comprise only copolymers of ethylene with alpha-olefins, usually 1-butene, 1-hexene or 1-octene and are recognized by those skilled in the art as having a high degree of linearity of structure with short branching rather than the long side branches characteristic of LDPE. However, VLDPEs have lower densities than LLDPEs. The densities of VLDPEs are recognized by those skilled in the art to range between 0.860 and 0.915 g/cm.sup.3 . A process for making VLDPEs is described in European Patent Document publication number 120,503 whose text and drawing are hereby incorporated by reference into the present document.
Various types of polyethylene resins have long been used to produce films having different properties. These polyethylenes have been used alone, in blends and with copolymers in both monolayer and multilayer films for packaging applications for such food products as poultry, fresh red meat and processed meat. In the food industry greater use of centralized processing of foods in conjunction with increased handling and long distance transportation have increased the demand for packaging films having superior properties.
In the poultry and meat segments of the food industry thermoplastic heat shrinkable flexible films are utilized to maintain freshness. Meat is frequently sold fresh, frozen or cooked; therefore films advantageously provide protection at various temperatures. Food items such as primal and subprimal cuts of beef, ground beef and processed meats are known to use coextruded, extrusion coated or laminated films which utilize such compositions as LLDPE, nylon, polyester, copolymer of vinylidene chloride (PVDC), ethylene-vinyl acetate copolymer (EVA) and ionomers.
It is generally known that selection of films for packaging food products includes consideration of one or more criteria such as puncture resistance, shrinkability, shrink force, cost, sealability, stiffness, strength, printability, durability, barrier properties, machinability, optical properties such as haze and gloss, flex-crack resistance and government approval for contact with food.
For example, several film materials containing polyethylene have been either used or proposed for packaging frozen poultry. In general, commercial poultry packaging operations require bags made from materials able to withstand the following typical process and transfer steps:
1. Inserting a bird into a bag fabricated from a shrinkable film; PA1 2. Evacuating the bag; PA1 3. Clamping or otherwise sealing the neck of the bag; PA1 4. Transporting the bird (e.g. by a conveyor belt) to a shrink tunnel; PA1 5. Shrinking the bag tightly around the bird by exposing the bag to a temperature of about 90.degree.-95.degree. C. for up to about six to eight seconds; PA1 6. Quick freezing and storage of the packaged bird at temperatures as low as -40.degree. C.; and PA1 7. Transporting the packaged bird from the commercial packer to the ultimate user. PA1 a) A shrinkage value that yields a reduction in the area of the film at a temperature from 90.degree.-95.degree. C. that is sufficient to conform the film to the irregular shape of the bird; PA1 b) a shrink force at a temperature of 90.degree.-95.degree. C. is required that is sufficient to pull the wings of the bird in tightly toward the body with sufficient residual shrink force to maintain a tight wrap around the bird; and PA1 c) a puncture resistance sufficient to withstand the packaging operation itself as well as subsequent transport of the packaged bird. PA1 (i) A dynamic puncture resistance greater than that for similarly made films comprising a copolymer of ethylene and a C.sub.6 -C.sub.8 alpha-olefin without 1-butene. PA1 (ii) A hot water puncture value of at least 20 seconds, preferably at least 60 seconds, and most preferably at least 120 seconds. PA1 (iii) A shrinkage value of at least about 15 percent in at least one direction, (preferably at least 20 percent in the machine direction) and desirably at least about 20 percent (preferably at least 25 percent and most preferably at least 30 percent) in the transverse direction.
A film useful for frozen poultry packaging will include among its desirable properties the following:
All the above properties should be provided in a film at a minimum of cost.
Several polyolefin films have previously been proposed for use as poultry bags.
U.S. Pat. No. 3,555,604 (Pahlke) discloses that low density polyethylene may be biaxially oriented to produce a film which is useful for packaging foodstuffs such as turkey.
Multilayer biaxially oriented films have been proposed for poultry bags such as those described in U.S. Pat. No. 3,900,635 (Funderburk, Jr. et al) wherein a first layer comprises an ethylene homopolymer or copolymer and a second layer comprises a blend of an ionomer and a second ethylene homopolymer or copolymer.
Also, various blends of different polyethylene resins have been reported. For example, blends of LLDPE with LLDPE or LDPE have been reported in the article by Utracki et al, "Linear Low Density Polyethylene and Their Blends: Part 4 Shear Flow of LLDPE Blends with LLDPE and LDPE", Polymer Engineering and Science, Vol. 27, No. 20, pp 1512-1522 (mid-November, 1987). In its introduction, the above article states that . . . "at least 60% of LLDPE is sold in blends with polyolefins or EVA (ethylene-vinyl acetate copolymers) (cite omitted). Amelioration of properties (e.g., puncture resistance), lowering of material cost or improvement of processability are the main reasons". The article goes on to discuss data relating to blends of a LLDPE made from a copolymer of polyethylene with 1-butene with (a) a LLDPE made from a copolymer of polyethylene and 1-hexene, and (b) a LDPE.
Various VLDPEs have been suggested for use as suitable resins for making a shrinkable multilayer or single layer film for food packaging.
U.S. Pat. No. 4,640,856 (Ferguson et al) discloses heat shrinkable multilayer films containing VLDPE which are useful in packaging meat, poultry and dairy products. Ferguson, et al in describing their thermoplastic polymeric layer also state that "in certain applications blends of VLDPE, LLDPE and/or EVA may be used to achieve desired properties".
Other patents have disclosed use of VLDPE resins in film including U.S. Pat. Nos. 4,671,987; 4,720,427; and 4,726,997.
Various ethylene based terpolymer resins having densities below 0.915 have been previously described. For example, EP Patent Application Publication No. 144 716 (Carrick et al) discloses a process where "ethylene is copolymerized with one or more comonomers which comprise 1-olefins having between 3 and 8 carbon atoms in their main carbon chains. The 1-olefin comonomers may be substituted or unsubstituted. Olefins such as propylene, 1-butene, 1-hexene, 1-octene and substituted comonomers such as 4-methyl-l-pentene-1 are preferred". Copolymers are said to be formed having "densities generally in the range of less than about 0.87 g/cc to about 0.94 g/cc." Although broadly suggesting a process for copolymerizing ethylene with one or more monomers, Carrick et al does not have any specific examples of copolymers made with more than two monomer components. Also, Carrick et al is silent regarding any utility of the materials disclosed therein for making heat shrinkable films, for example, for packaging.
The concept of using copolymer resins having more than two monomers to form heat sealable films has been broadly disclosed, for example, in European Patent Application Publication number 247,897 (Bossaert et al). Bossaert et al disclose films which are preferably based on propylene which are heat shrinkable, and may be biaxially oriented. These films are described as being useful for packaging. Bossaert et al are silent regarding any puncture or shrinkage properties of their film and do not have any specific examples of copolymers made with more than two monomer components.
Heat shrinkable films comprising propylene-ethylene-alpha-olefin terpolymer are also known as shown by Japanese Patent Application Publication Number 45306/1988 (Isaka et al). Isaka et al disclose a propylene-ethylene-alpha-olefin terpolymer heat-shrinkable film. This terpolymer film is described as containing less than ten weight percent ethylene.
Also, various government regulatory approvals for various terpolymers for use in contact with food have been or are being sought for such terpolymers as ethylene-octene-butene terpolymer, ethylene-octene-hexene terpolymer (See e.g. Fed. Reg. 23798 Jun. 24, 1988) or ethylene-hexene-butene terpolymer (See 21 CFR 177.1520).
None of the foregoing publications have disclosed biaxially stretched, heat shrinkable films made from a very low density polyethylene terpolymer of ethylene, 1-butene, and a C.sub.6 -C.sub.8 alpha-olefin, or ethylene, 1-hexene and either a C.sub.6 -C.sub.8 alpha olefin. Also, presently known films used as poultry bags continue to suffer from insufficient puncture resistance, and/or shrinkability.
Puncture resistance is a useful property of packaging films in general and an important property of food packaging films. Puncture resistance is very important for films used in forming bags for poultry. These poultry bags must have a high puncture resistance in order to withstand packaging operations and transport as well as retail customer inspection and handling. Punctured poultry bags not only expose the contained birds to spoilage agents, but also allows leakage of liquid from within the bag. This leakage is highly undesirable to grocery shoppers and retailers. In retail poultry displays, leaked liquid often is transferred to adjacent products making displays and selection messy. A shopper who places a punctured bag into a grocery cart may cause moisture damage to paper products or packaging. In addition, concern about possible salmonella or other bacterial contamination via contact with leaked poultry liquid increases the desirability of puncture resistant poultry packaging.
Punctured and leaking bags are still very much a problem in poultry packaging. Recently, very low density polyethylene (VLDPE) resins have been utilized in making shrinkable packaging films including films for food contact packaging.
One type of commercially available VLDPE is a copolymer of ethylene and 1-butene sold by Union Carbide Corporation under the brand designation DFDA 1137, Natural 7. Disadvantageously, this resin has been found to have low puncture resistance in packaging operations. In particular, where packaging films are exposed to elevated temperatures in a film shrinking step of a packaging operation, puncture resistance is undesirably low.
Another type commercially available VLDPE is a copolymer of ethylene and 1-octene sold by Dow Chemical Company under the brand designation Attane 4001. While this film has improved puncture properties relative to DFDA 1137, it has undesirably low shrinkage values.
An experimental VLDPE that is a copolymer of ethylene and 1-hexene was obtained from Union Carbide Corporation under the experimental brand designation DEFD 1569. In one experiment disclosed in the present application, heat shrinkable, biaxially oriented films were made under similar conditions. A film made from this experimental ethylene, 1-hexene VLDPE when compared to a film made from DFDA 1137, had a similar dynamic puncture resistance, greater hot water puncture resistance and undesirably low shrinkage values.
Advantageously, a biaxially stretched, heat shrinkable film of the present invention may have both high dynamic puncture resistance relative to similarly formed films made from commercially available 1-butene based VLDPE resins and experimental 1-hexene based VLDPE resins as well as high shrinkage values relative to similarly formed films made from commercially available 1-octene based VLDPE resins and experimental 1-hexene based VLDPE resins. An inventive film has also been found to have a high puncture resistance at elevated temperatures (hot water puncture resistance) relative to a similarly formed 1-butene based VLDPE film.
Although the broad concept of making ethylene-alpha olefin terpolymer resins has been previously disclosed in the art, heat shrinkable, biaxially stretched films of the specific terpolymers according to the present invention have not been taught in the prior art. Previously, where specific terpolymers have been disclosed in any detail, most often propylene and/or a diene has been one of the terpolymer comonomers. These known terpolymer resins are generally synthetic elastomers having properties similar to rubber, which makes these materials generally undesirable for use as the principal component of the packaging film. These elastomeric resins typically have very low crystallinity to the point of being amorphous with no definite crystalline melting point unlike the resins utilized in the present invention. Moreover, the utility and properties of biaxially stretched, heat shrinkable, flexible films comprising the specific ethylene, C.sub.6 -C.sub.8 alpha-olefin, and 1-butene or 1-hexene terpolymers according to the present invention have not been previously disclosed. These previously unknown, useful and surprising properties of these novel films are now disclosed below for the first time in the present specification.